Present selective electrodes having an organic membrane are used, in particular, for measuring the following cations: hydrogen; sodium; potassium; calcium; lithium; magnesium; and barium; or the following anions: nitrate; fluoborate; chloride; and carbonate. They are applied, for example, to measuring the concentrations of some of these ions in biological fluids (blood, serum, plasma, urine). They are generally constituted by five essential parts shown in FIG. 1 which is a diagram illustrating the prior art:
an electrode body 1 generally constituted by a tube of polymer that does not conduct electricity; PA1 a sensitive membrane 2 constituted by a dispersion of a sensitive ion-conducting material optionally together with a conducting salt in a thin film which is generally constituted by polyvinyl chloride including a plasticizer; PA1 an internal solution 3 which may optionally be gelled, containing a known constant concentration of the ion to which the electrode is sensitive, and generally also a known constant concentration of chloride ions; PA1 an internal reference element 4 generally constituted by a piece of silver coated with silver chloride and immersed in the internal solution 3; and PA1 an electrode head 5 for passing a screened connection cable 6 or equipped with a connector enabling such a cable to be connected thereto. PA1 easily made at low cost in any configuration suitable for the intended application; PA1 resistant to physical, thermal, chemical, and mechanical constraints that may be applied to it; PA1 compact; PA1 easily put into place or positioned at will by an operator while a measurement is being performed; and PA1 perfectly sealed. PA1 the body is a matrix body made of a single type of material which does not conduct electricity, which is chemically inert, and which has the ability to be filled with various different materials in the dispersed, dissolved, or grafted state; PA1 the sensitive zone of the matrix body comprises: PA1 the conductor passes through a portion of the matrix body in sealed manner and is inserted into the underlayer.
Such selective electrodes of a design which is now conventional are made using a technology which is difficult and expensive to implement but which is suitable for satisfying an important constraint, namely that the various different materials must be suitable for being assembled together even though at least some of them are in the presence of a saline aqueous solution. The assembly must conserve effective sealing throughout the lifetime of the electrode under various environmental conditions.
Further, these electrodes are rather fragile and their shape as well as their positioning in use cannot be selected freely by the manufacturer or by the user.
Getting rid of the internal gell or solution and the reference element would naturally represent a considerable simplification. Unfortunately, electrochemists are aware that a direct metallic contact on the rear face of the sensitive membrane is not feasible when the charges moving through the membrane are constituted solely by ions. In this case the transfer of charge between ions and electrons is poorly defined since there is no stable oxydo-reducing system at the interface, with the interface, in addition, presenting a sharp phase transition of uncertain geometry. This difficulty is also encountered with glass membrane pH measuring electrodes for which an internal solution and reference element continue to be universely employed.
Several authors have described selective electrodes in which solid contacts are used. In this context, reference may be made to the excellent bibliography of B.P. Nikolskii and E.A. Materova in "Solid-contact in membrane ion selective electrodes" published in "Ion Selective Electrodes Review", 1985, vol. 7, pp. 3-39, which brings together the descriptions and the results which have been obtained.
Some publications or inventions mention electrodes making use of an intimate mixture of an ionic conductor and of an electronic conductor or an oxydo-reducing equilibrium in a polymer membrane. Mention may be made in this context of Russian patent number 898314 (1981). Solid contact electrodes based on the use of carbon have also been described, in particular by D. Midgley and D.E. Mulcaby in "Carbon substrate electrode" published in "Ion Selective Electrode Reviews" vol. 5, no. 2, 1983, pp. 165-241. It appears that such systems should be rejected because they give rise to interference from the oxydo-reducing species present in the measurement solutions.
The use of silicone elastomer for constituting a composite electrode has also been mentioned in French patent number 79 28 651 (2 469 202). The "composite electrodes" described in this patent are constituted by dispersions of mixtures of inorganic species distributed in a way which satisfies no precise configuration. Further, none of them makes use of a filler of a species sensitive to ions and having purely ionic conductivity.
Mention may also be made of an article by R.W. Cattrall and I.C. Hamilton published in "Ion Selective Electrodes Review", vol. 6, no. 2, 1984 entitled "Coated wire ion selective electrodes" and relating to selective electrodes comprising a metal support directly coated with a polymer film incorporating an electro-active conductor.
Electrodes of this type are heterogeneous in character and do not give satisfaction because of the sharp and ill-defined transition between two very different solid phases constituted by a noble metal and by a polymer film. This gives rise to instability in electrical continuity at the interface. The same remark applies to French patent number 2 262 302 which describes an electrode based on depositing a membrane of silicone with an ionophore filler directly deposited on a plate of graphite.
Mention may also be made of the constructive proposal made by Jose L.F.C. Lima and Adelio A.S.C. Machado in the publication "Analyst", July 1986, vol. III. In this proposal, a selective electrode is constituted by a membrane of PVC including a filler of a sensitive species and covering a solid support made of conductive epoxy. It appears that such a structure is still not satisfactory because of the large difference in nature between the two solid materials respectively constituting the electronic conductor element and the ionic conductor element, with the result that there is considerable potential drift as indicated in the comparative example which appears in the present patent specification.
Such structures,, likewise heterogeneous in character, also suffer from the problem of sealing which is difficult to solve in the long term.
Finally, ion selective sensors having an inorganic sensitive element of the compressed pellet type and having direct electrical contact, such as those sold by numerous manufacturers, all have combined electrical conduction (i.e. both ionic and electronic) and this gives rise to interference from oxydo-reducing systems.
The present invention seeks to remedy the above drawbacks and proposes a new potentiometic sensor having the advantages of being: